Many processes have been proposed for forming a dye image (color image) by thermal development. With a process of forming a color image by binding an oxidation product of a developing agent with a coupler, U.S. Pat. No. 3,531,286 discloses a combination of a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler, U.S. Pat. No. 3,761,270 discloses p-aminophenol type reducing agents, Belgian Patent No. 802,519 and Research Disclosure, Sept. 1975, pp. 31-32 disclose sulfonamidophenol type reducing agents, and U.S. Pat. No. 4,021,240 discloses a combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler.
However, these processes have the defect that silver-stained color images result since an image of reduced silver and a color image are formed at the same time in exposed portions after thermal development.
In order to overcome this defect, it has been proposed to remove the silver image by some type of liquid processing, or to transfer only the dye image onto a sheet having other layers such as an image-receiving layer. The latter process, however, has the defect that it is difficult to discriminate the dye from an unreacted reactant and transfer only the dye.
Furthermore, the above-described processes involve the defects that they generally require a comparatively prolonged developing time and that resulting images have a seriously high fog and a low image density.
In order to overcome these defects, the following processes have been proposed, for example: A process of imagewise releasing a mobile dye by heat and transferring this mobile dye to a dye-immobilizing element having a mordant with the aid of a solvent such as water; a process of transferring the dye to a dye-immobilizing element with the aid of a high-boiling organic solvent; a process of transferring the dye to a dye-immobilizing element with the aid of a hydrophilic thermal solvent incorporated in the dye-immobilizing element; and a process of using a heat-diffusible or heat-sublimable dye as the mobile dye and transferring it to a dye-receiving material such as a support (U.S. Pat. No. 4,463,079, U.S. Patent 4,474,867, U.S. Pat. No. 4,478,927, U.S. Pat. No. 4,507,380, U.S. Pat. No. 4,500,626, U.S. Pat. No. 4,483,914, JP-A-58-149046 (the term "JP-A") as used herein means an "unexamined published Japanese patent application"), JP-A-58-149047, JP-A-59-152440, JP-A-59-154445,JP-A-59-165054,JP-A-59-180548,JP-A-59-168439, JP-A-59-174832,JP-A-59-174833,JP-A-59-174834,JP-A-59-174835, etc.).
In the above-described image-forming processes, a light-sensitive element having at least a light-sensitive silver halide, a binder, and a dye-providing substance is used for producing or releasing a mobile dye, and a dye-immobilizing element having at least one dye-immobilizing layer is used for receiving and immobilizing the produced or released mobile dye. However, in a low-moisture environment, the light-sensitive element and the image-immobilizing element can curl in such a manner that both edges thereof become higher than the horizontal plane when placed with the light-sensitive layer or the dye-immobilizing layer upward, since the hydrophilic colloid contained in the light-sensitive layer or the dye-immobilizing layer contracts in the low-moisture environment. On the other hand, in a high-moisture environment, the light-sensitive layer or the dye-immobilizing layer stretches because of the absorption of moisture by the hydrophilic colloid contained in the light-sensitive layer or the dye-immobilizing layer and, when placed with the light-sensitive layer or the dye-immobilizing layer downward, the light-sensitive element or the image-immobilizing element can curl in such a manner that both edges thereof become higher than the horizontal plane.
When a curled light-sensitive element or a curled dye-immobilizing element is superposed on each other, there results an insufficient contact therebetween because of an uneven superposition or presence of air between the two elements. As a result, the development or transfer is partially delayed, leading to the formation of uneven images with deteriorated image quality.
In order to remove this curling, a backing layer is coated on the back side (opposite side to the side on which a light-sensitive layer or a dye-immobilizing layer exists) of the support of a light-sensitive element or a dye-immobilizing element. However, while the light-sensitive layer or the image-immobilizing layer undergoes stretching of the coated layers while being conveyed in a heat-developing machine because the layers are wet by a transfer solvent such as water, the backing layer tends to shrink since it is heated by heating rollers or the like and becomes dried.
In accordance with the present invention, it has been found that the above problems can be prevented by selecting a proper stiffness of a support of a light-sensitive element and/or a support of a dye-immobilizing element.
That is, in a system where heating is conducted with simultaneously conveying materials to be heated, for example, a heating apparatus using multi-stage heating rollers, curling of the light-sensitive element or the dye-immobilizing element can not be prevented during heating, since the heating apparatus is not of a type in which heating is conducted with pressing the light-sensitive element and the dye-immobilizing element throughout the heating processing.
If the light-sensitive element or the dye-immobilizing element curls during heating, air gets into the space formed between the two elements to inhibit the transfer of dyes or generates a fold at the front end of the light-sensitive element and/or the dye-immobilizing element in the direction of the conveyance of the elements.